Principles of Heredity 139 



are, and consequently not belonging to the next generation 

 at all. If then any changes take place in it consequent on 

 fertilisation, they are to be regarded not as in any sense a 

 transmission of character by heredity, but rather as of the 

 nature of an "infection." If on the other hand it is desired 

 to study the influence of hereditary transmission on seed- 

 coat characters, then the crossed seeds must be sown and 

 the seed-coats of their seeds studied. Such infective changes 

 in maternal tissues have been known from early times, a 

 notable collection of them having been made especially by 

 Darwin ; and for these cases Focke suggested the convenient 

 word Xenia. With this familiar fact I would not for a 

 moment suppose Professor Weldon unacquainted, though it 

 was with some surprise that I found in his paper no reference 

 to the phenomenon. 



For as it happens, xenia is not at all a rare occurrence 

 with certain varieties of peas ; though in them, as I believe 

 is generally the case with this phenomenon, it is highly 

 irregular in its manifestations, being doubtless dependent 

 on slight differences of conditions during ripening. 



The coats of peas differ greatly in different varieties, 

 being sometimes thick and white or yellow, sometimes 

 thick and highly pigmented with green or other colours, 

 in both of which cases it may be impossible to judge the 

 cotyledon-colour without peeling off the opaque coat ; or 

 the coats may be very thin, colourless and transparent, so 

 that the cotyledon-colour is seen at once. It was such a 

 transparent form that Mendel says he used for his experi- 

 ments with cotyledon-colour. In order to see xenia a pea 

 with a pigmented seed-coat should be taken as seed-parent, 

 and crossed with a variety having a different cotyledon- 

 colour. There is then a fair chance of seeing this 

 phenomenon, but much still depends on the variety. For 



